As illustrated in FIG. 11, an impeller (a compressor impeller) 1 included in various hydraulic machines or pneumatic machines such as liquid pumps or electric generators includes blades 2, and an impeller disk 3 and an impeller cover 4 disposed such that the blades 2 are interposed therebetween.
The impeller disk or the impeller cover is formed into a truncated cone shape (a disk shape), using die forging, roll forging, or the like.
Specifically, when forming an impeller disk or the like using die forging, for example, a material (a rough forged material) extracted from a furnace is inserted into a central hole of a die having a predetermined shape and tapped, and the low-temperature material is placed in a furnace and reheated. By repeating insertion into the die and tapping, the material is gradually pushed out in a radial direction to finish the material into a desired shape.
When using roll forging, for example, a material extracted from the furnace is placed on a table of the forming device, the material is pressed by a forming roll, and by rotating the table, the material is gradually pushed out in the radial direction and is formed in a truncated cone shape. Further, by relatively moving the forming roll in the radial direction with respect to the table, and repeating reheating in which the low-temperature material is placed in the furnace, and roll forging, the material is finished into a desired shape (for, example, see Patent Literature 1).